Professor Patrick Marshall's important and interesting memoir, which has recently appeared, on the geology of Mangaia, one of the Cook Islands, inevitably suggests a comparison between this island and Rurutu in the Austral Group, which I visited in 1925, during the St. George Expedition to the Pacific, and described in a recent communication to the Geological Society. Owing to the shortness of the Expedition's stay at this island, my observations were necessarily somewhat incomplete, but I had the advantage of many conversations with Mr. S. Russell, who spent several months there in 1924, prospecting for minerals. He gave me copies of his reports, and he accompanied us on our visit.